Carbon fibers are used as reinforcing materials in a variety of applications ranging from aerospace to sporting goods. In order to achieve improved mechanical properties, these fibers are typically used as continuous materials either in prepreg form or as filament wound materials to produce composite articles. However, chopped fibers, either sized or unsized, are also used in order to reduce cost, in applications where less than optimal mechanical properties are acceptable or in applications where other physical properties, such as electrical properties, are important.
One of the problems associated with handling unsized carbon fibers is their tendency to separate from one another. As a consequence, they cannot be chopped into shorter lengths without "fluffing". The result is that the dry, chopped fibers have a very low bulk density. In many uses this low bulk density is a disadvantage, since it makes the material hard to handle, hard to package, and hard to incorporate or mix with other materials.
The fibers are therefore typically sized with a nonvolatile sizing agent that holds the fibers together and protects the bundle both during and after chopping. This sizing agent is generally chosen to be compatible with the resinous matrix material that the fibers will reinforce and as such, the sizing becomes a part of the chopped fiber bundle and composite article. Since the sizing remains on the fiber, the fiber bundles are not easily dispersed until they are added to the matrix resin and then only if the sizing agent and matrix resins are compatible.
Other methods for holding the chopped fiber bundles together have also been disclosed. For example, U.S. Pat. No. 4,003,773 discloses passing a continuous tow through a bath of liquid impregnating material at an elevated temperature. After controlling the amount of material applied to the tow, the tow is chopped into discs of desired thickness and the impregnating material is removed by dissolving it in a solvent. U.S. Pat. No. 3,921,874 discloses forming chopped fiber bundles by impregnating the continuous fiber tows with a liquid such as water, collimating the fiber bundles, freezing the liquid, and chopping the fiber into discrete bundles. After chopping, the impregnant is melted and the fiber bundles are dried to remove the liquid. U.S. Pat. No. 5,030,435 teaches a process for producing chopped pitch-based carbon fiber strands which comprises coating a pitch strand with a low-boiling solvent such as water or methanol, cutting the pitch strand to a predetermined length, then carbonizing the chopped strands in an inert atmosphere. Bundles of chopped fibers exhibiting high bulk densities are reported.
In addition to being used as reinforcing agents in composite structures, chopped carbon fibers can be dispersed in air to obscure military vehicles from millimeter-wave radar. These chopped fibers must be packaged for subsequent dispersal, and the volume of the package is critical for handling large amounts of short fibers. If the packing density is low, too much volume is occupied and the fiber cannot be transported within the limited confines of a military vehicle.
Packages of chopped fibers that are more densely packed than those presently available would be valuable, since the package volume could be reduced dramatically and many more pounds of fiber could be carried in a limited volume. The fibers would also have to be easily dispersible, either in air or in another medium such as a matrix resin used in the manufacture of composite structures.